Detecting and measuring interference contained within a digital carrier

ABSTRACT

A method and system for measuring noise and or interference in a communications signal without taking the signal out of service. In the present invention the communications signal is converted from an RF signal into an IF signal. The IF signal is then digitized and stored. The stored signal is processed to determine the interference signal. The interference signal is calculated from an error vector produced by a blind equalizer demodulator. The interference signal is extracted and presented to a user.

[0001] This application claims the benefit of U.S. ProvisionalApplication Serial No. 60/361,493, filed Mar. 4, 2002, the contents ofwhich are hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] The invention relates to detection of interference and noise in atransmitted signal.

DESCRIPTION OF THE RELATED ART

[0003] Interference (including noise) showing up in-band to atransmitted carrier is a common problem in wireless communicationsystems. For example, in satellite communication systems, interferencecan be caused by, but is not limited to, isolation degradation ofcross-polarized signals, adjacent satellite traffic, locally receivedterrestrial signals, or an unauthorized transmission. In many cases,interference can be very difficult to detect, however, its impact on thereceive quality of the transmitted digital carrier can be significant.

[0004] The most common approach to determining the presence ofinterference is to temporarily remove the service (the transmittedcarrier) and inspect the received power spectrum with a frequencyanalysis device such as a spectrum analyzer. Although this approach canbe effective, it causes a service interruption that can last for manyhours. In some cases, interference is not the problem, and the servicewas interrupted unnecessarily.

BRIEF SUMMARY OF THE INVENTION

[0005] The invention includes a method of and an apparatus for detectingand measuring noise and interference, which is in-band to a receivedcommunications carrier. To alleviate drawbacks to conventionalapproaches, the applicant has developed a non-intrusive interferencedetection and noise measurement approach. With this approach,interference and noise can be detected and measured without taking thecarrier out of service. Rather, the measurements are made while thecommunications circuit (the transmitted carrier) is active.

[0006] In one aspect, in-band interference in a carrier signal in acommunication system is detected. A signal is acquired including thecarrier signal and an interfering signal. The interfering signal isextracted from the carrier without interrupting the carrier.

[0007] In another aspect, a signal is received, filtered, and digitized.Decimation is then performed and the signal resampled. Blindequalization and demodulation are performed thereby forming an errorvector that is representative of the interference signal.

[0008] In yet another aspect, a receiver acquires a digital signal. Asignal processor conditions the digital signal, and a blind equalizerdemodulator forms an error vector that is representative of aninterference signal included in the carrier signal.

[0009] These and other aspects are described in more detail herein.

BRIEF DESCRIPTION OF DRAWINGS

[0010]FIG. 1 illustrates a flow diagram showing the main processingassociated with this invention;

[0011]FIG. 2 shows a detailed flow diagram of the processing associatedwith this invention; and

[0012]FIG. 3 illustrates a system in accordance with an embodiment ofthe present invention; and

[0013]FIG. 4 shows a graphical display that might be displayed to a userof this invention.

DETAILED DESCRIPTION OF INVENTION

[0014]FIG. 1 shows a flow diagram of a process 100 in accordance with anaspect of the present inveniton. A signal, which contains noise andinterference, is received in a step 101. The signal is down-converted toan intermediate frequency (IF) and then digitized by a sampling device,in an acquisition step 102. The digital samples are stored in memory forfurther processing, also in the acquisition step 102. Using the storedsamples, the signal is processed to create a re-sampled baseband versionof the received signal, in a digital formatting step 103. Using thisre-sampled baseband signal, an equalized error signal is created, in aninterference processing step 104. This error signal is further processedto create a power spectrum of the underlying noise and interferencecontained in the received carrier, also in the interference processingstep 104. This power spectrum of the noise and interference can bemeasured for interfering signals as well as displayed to the user, in anoutput step 105.

[0015]FIG. 2 is a more detailed flow diagram of a process 200 inaccordance with an embodiment of the present invention. An input signalmay be a radio frequency (RF) signal from an antenna. Alternatively theinput signal may be any communication signal in any frequency band i.e.RF, IF, microwave or optical.

[0016]FIG. 3 illustrates a system 400 in accordance with an embodimentof the present invention. The system 400 detects and measures in-bandinterference and noise in an input signal 407 in accordance with themethod of FIG. 2. An input signal 407 is received from a satellite 401by a satellite dish 402. The input signal 407 may be transmitted bymeans other than the satellite 401, including but not limited to a radiotransmitter, a cable transmitter, a cell tower, a microwave transmitter,or an optical transmitter. The input signal 407 may be received by meansother than the satellite dish 402, including but not limited to anantenna, a microwave dish, or an optical receiver. The present inventionis applicable to any system that communicates a digital signal from atransmitter to a receiver, regardless of the medium or the carrierfrequency.

[0017] Referring to FIGS. 2 and 3, a receiver 403 may convert the inputsignal 407 from an RF input signal 407 to an IF signal in a step 201.The IF signal may be at any frequency that makes the following detectionsimpler, cheaper, or more accurate. The receiver 403 may further filterand adjust the level of a signal, which is representative of the inputsignal 407 in a step 202. The receiver 403 may filter the input signal407 with a band-pass filter to limit the input signal 407 or itsrepresentative to the carrier and its modulation. An amplifier withautomatic gain control may adjust the level of the filtered input signal407 or its representative, also in the step 202. The input signal 407may be amplified to take full advantage of an A/D converter in thereceiver 403. The A/D converter is expected to perform best when thefull amplitude bandwidth is used.

[0018] The A/D converter, in the step 203, produces a digitized versionof the filtered IF signal. The A/D converter may sample the IF signal ata frequency at least twice the frequency of the highest frequency ofinterest in accordance with Nyquist's theorem, though another samplingfrequency may be used. The digitized version of the IF signal is thenstored as a snapshot 408 in a step 204. The above steps 201-204 maycomprise the acquisition step 102 of FIG. 1.

[0019] A signal processor 404 may analyze the snapshot 408 to calculateparameters representative of the input signal 407 including, a bandwidthof the input signal 407, a center frequency of the input signal 407, asymbol rate of the input signal 407, amplitudes of the carrier lines,frequencies of the carrier lines, and maximums of the carrier lines.

[0020] In a next step 205 of the process 200, a power spectrum of thesnapshot 408 is calculated by the signal processor 404. Multiple powerspectrums may be calculated and averaged together, to create a spectraldensity periodogram. The power spectrum may be calculated usingconventional Fast-Fourier Transform (FFT) methods, for converting the IFsignal from time space to complex frequency space. Other methods besideFFT may be used to convert the IF signal to frequency space. The powerspectrum or the spectral density periodogram may be displayed to user atthis time.

[0021] The input signal may be a modulated carrier. The center frequencyand bandwidth (BW) of the carrier may be calculated by a signalprocessor 404 in a step 206 using the power spectrum or the spectraldensity periodogram of the IF signal from step 205. If the centerfrequency and the bandwidth are already known, however, then the steps205 and 206 may be skipped.

[0022] Once the center frequency of the carrier is known,down-converting of snapshot 408 to the baseband of the carrier may beperformed in step 207 by the signal processor 404. The snapshot 408 maybe further filtered such that the signal is limited in bandwidth to thatof the baseband signal, also in step 207. Further, the snapshot 408 maybe decimated also in a step 207. Decimation may be performed at afrequency at least twice the frequency of the highest frequency ofinterest in accordance with Nyquist's theorem.

[0023] The carrier signal may have multiple carrier lines. In a step208, information about the carrier such as symbol rate, and estimates ofthe amplitude and frequency of the carrier lines may be calculated. Thisinformation may be calculated by performing magnitude, square, cube andquad power transforms on the signal and recovering the maximums.

[0024] The estimates of the amplitude and frequency of the carrier linesmay be used to determine the modulation of the digital carrier and thefrequency in a step 209. By inspecting maximums of the carrier lines,the modulation of the carrier may be determined. Using information aboutthe carrier frequency, any down-conversion error in the decimated signalmay be removed in a step 210. For example if the baseband signal isoffset, it may be recentered such that any offset in the carrierfrequency is removed.

[0025] In a step 211, the carrier signal may be re-sampled by the signalprocessor 404, such as at a sample rate of two samples per symbol, and aresampled signal may be an output. This sample rate may be determinedfrom the symbol rate calculated in the step 208. The above steps 205-211may be performed in the digital formatting step 103 of FIG. 1.

[0026] A blind equalizer demodulator 405 may calculate an error vectorthat is representative of the interference signal in the input signal407 in a step 212. This step produces an error vector that may be usedto calculate the interference signal that is in the input signal 407. Adigital communication system modulates a carrier wave for transmittingsymbols to a receiver. In such a digital system, each symbol hasdiscrete levels of amplitude and/or phase at which the carrier ismodulated. A goal of the demodulator 405 is to determine the levels atwhich the carrier is modulated. It does this by making a first initialguess of the modulation levels and then calculating an error vector thatrepresents the difference between the initial guess and the measuredsignal. Then the guessed modulation levels are adjusted to minimize anerror function based on the error vectors. The guessed modulation levelsare continuously adjusted until the error function has been minimized,at which point the modulation has converged. There are many ways toadjust the levels, including decision directed least mean square(DD-LMS) and constant modulus algorithm (CMA), both of which are wellknown in the literature. Conventionally the blind equalizer demodulator405 is used to calculate the symbols in the input signal 407. Here, theoutput of interest is the error vector as opposed to the prior art wherethe output of interest is the symbols.

[0027] In a step 213, a first M samples are removed from the errorvector to produce a new error vector. Depending on the quality of theinitial first guess, the first M samples may have large error vectorsthat do not truly represent the noise and interference in the inputsignal 407. Before the blind equalizer demodulator 405 converges in thestep 212, the first M samples of the error vector may contain errors.The DC bias of the new error vector is removed in a step 214, bysubtracting the mean of the new error vector from the new error vectorto produce an in-band vector that is representative of noise andinterference in-band to the carrier. Any processing artifacts may alsobe removed in the step 214. The power spectrum of the in-band vector iscalculated to convert the complex time representation of the in-bandvector into a frequency domain representation, in a step 215. In a step216 the spectral properties of the in-band vector are measured such ascenter frequency, BW, power, C/N and detected interference energy. Theabove steps 212-216 may comprise the interference processing step 104 ofFIG. 1 preformed by signal processor 405.

[0028] A power spectrum of the error vector, such as a trace 302 in aFIG. 4, may be sent to a display 406 for presentation to a user in astep 105. A power spectrum of the input signal 407, such as trace 301 inFIG. 4, may also be sent to the display 406 also in step 105.Furthermore, the spectral properties of the error vector may be sent tothe display 406. Other calculations regarding the digital signal mayalso be sent to the display 406.

[0029] The system described in FIG. 3 may also implement all the stepsin the flowchart 200 of FIG. 2 and Table 1 as follows. The system may beimplemented in hardware and/or software. The system may be implementedin a standard PC and/or may be implemented with custom electronics.

[0030] Table 1 presents the method of FIG. 2 in tabular format. TABLE 1Functional Block Input Description Output RF Down-conversion FromAntenna Convert RF signal to IF representation of (201) IF Frequencysignal Filtering and Gain Control IF Signal Band-limit signal andFiltered and amplified (202) adjust signal level for signal A/DConverter A/D Conversion (203) Filtered IF Signal Convert analog signalSamples of IF signal at IF to digital samples Snapshot Memory (204)Samples of IF Signal Store IF samples Samples of IF signal PSDComputation (205) Samples of IF Signal Convert time domain Powerspectrum representation of signal to frequency domain representation.Spectral Detection and Power Spectrum Detect and measure Centerfrequency and Measurement (206) carrier of interest BW of carrier toanalyze Down-convert and Samples of IF signal, Down-convert carrierDecimated signal decimate (207) center frequency and to baseband, filterand (complex signal BW estimation decimate representation) and decimatedsample rate Carrier and Baud Decimated Signal Perform magnitude, Symbolrate, Recovery (208) square, cube and estimates of quad power amplitudeand transforms on signal. frequency of carrier Recover maximums linesModulation Identification Estimates of amplitude Determine ModulationModulation of digital (209) and frequencies of of digital carrier bycarrier and Carrier carrier lines inspecting carrier line frequencymaximums Carrier Correction (210) Decimated signal and Remove Decimatedsignal Carrier frequency down-conversion error from decimated signalRe-sampler (211) Decimated signal, Re-sample carrier to 2 Re-sampledsignal Decimated sample samples/symbol rate and symbol rate of carrierEqualizer/Demodulator Resampled Signal Equalize and Estimated symbols(212) demodulate signal and Error vector using blindequalizer/demodulator approach Remove M Initial samples Error VectorRemove first M New Error Vector (213) samples from Error vector. First Msamples contain errors from matched filter error DC bias removal (214)New Error Vector Remove mean from In-band vector New Error Vector(represents noise and interference in-band to carrier) PSD Computation(215) In-band vector Convert complex time In-band spectrum domainin-band vector to a frequency domain representation Spectral detectionand In-band Spectrum Detect and measure In-band spectral measurement(216) any spectral energy measurements (Center frequency, BW, power, andC/N of any detected interference energy) Display (217) In-band Spectrumand Display spectrum and Done In-band spectral measurement resultsmeasurements to user

[0031]FIG. 4 shows a graphical display 300 of data that the inventionmay present to a user. The trace 301 represents the power spectrum ofthe received carrier, and the trace 302 represents the power spectrum ofthe noise and interference, which are in-band to the received carrier.In this example, an interfering signal can be seen in the trace 302,which is not visible in the received carrier trace 301.

[0032] Thus, a technique has been described for detecting and measuringinterference within a digital carrier. The process can be completelyblind, meaning that the process described above will work even when thedigital carrier's RF and modulation parameters are unknown. The processdescribed detects the RF carrier, measures its RF and modulationparameters, equalizes and demodulates the digital carrier, extracts anerror vector, converts this error vector into a complex basebandestimate of the noise and interference. From this estimate, a powerspectrum of the in-band noise and interference is created. This powerspectrum is analyzed for spectral energy. The in-band spectrum andmeasurement results are displayed for a user.

[0033] While the foregoing has been with reference to particularembodiments of the invention, it will be appreciated by those skilled inthe art that changes in these embodiments may be made without departingfrom the principles and spirit of the invention, the scope of which isdefined by the appended claims.

What is claimed is:
 1. A method of detecting in-band interference in acarrier signal in a communication system comprising the steps of:acquiring a signal including the carrier signal and an interferingsignal; and extracting the interfering signal from the carrier withoutinterrupting the carrier.
 2. The method according to claim 1, withoutknowledge of the carriers RF or modulation parameters.
 3. The methodaccording to claim 1, with knowledge of the carriers RF or modulationparameters.
 4. The method according to claim 1, wherein the interferingsignal comprises noise.
 5. The method according to claim 1, wherein thecommunication system performs as a wireless communication system.
 6. Themethod according to claim 1, wherein the communication system performsas a satellite communication system.
 7. The method according to claim 1,wherein the carrier comprises an RF signal.
 8. A method of detectingin-band interference in a carrier signal in a communication systemcomprising the steps of: acquiring a digital signal including receivingthe signal, filtering the signal and digitizing the filtered signal;formatting the digital signal including filtering the digital signal,performing decimation and re-sampling of the digitized signal; andperforming blind equalization and demodulation thereby forming an errorvector that is representative of the interference signal.
 9. The methodaccording to claim 8, wherein the step of formatting the digital signalis performed without knowledge of the carriers RF or modulationparameters.
 10. The method according to claim 8, wherein the step offormatting the digital signal is performed with knowledge of thecarriers RF or modulation parameters.
 11. The method according to claim8, wherein the interfering signal comprises noise.
 12. The methodaccording to claim 8, wherein the communication system performs as awireless communication system.
 13. The method according to claim 8,wherein the communication system performs as a satellite communicationsystem.
 14. The method according to claim 8, wherein the carriercomprises an RF signal.
 15. The method according to claim 8, whereinacquiring the digital signal includes converting the carrier signal intoan IF signal, wherein the IF signal is representative of the carriersignal.
 16. The method according to claim 8, wherein filtering thedigital signal includes filtering the carrier signal to limit thebandwidth to a signal of interest.
 17. The method according to claim 8,wherein acquiring a digital signal includes storing a digitized filteredsignal into memory.
 18. The method according to claim 8, whereinformatting the digital signal includes calculating an average basebandand an average bandwidth of the carrier signal.
 19. The method accordingto claim 8, wherein filtering the digital signal includesdown-converting the digital signal to the baseband of the carriersignal.
 20. The method according to claim 8, wherein the re-sampling ofthe digital signal occurs at twice the frequency of interest.
 21. Themethod according to claim 8, including removing a M initial samples ofthe error vector, wherein the M initial samples occur while the blindequalization and demodulation is sill converging.
 22. The methodaccording to claim 8, including removing a DC bias from the errorvector.
 23. A system of detecting in-band interference in a carriersignal in a communication system comprising: a receiver for acquiring adigital signal a signal processor for conditioning the digital signaland a blind equalizer demodulator forming an error vector that isrepresentative of an interference signal included in the carrier signal.24. The system according to claim 23, wherein the signal processor iswithout knowledge of the carriers RF or modulation parameters.
 25. Thesystem according to claim 23, wherein the signal processor knows of thecarriers RF or modulation parameters.
 26. The system according to claim23, wherein the interference signal comprises noise.
 27. The systemaccording to claim 23, wherein the communication system performs as awireless communication system.
 28. The system according to claim 23,wherein the communication system performs as a satellite communicationsystem.
 29. The system according to claim 23, wherein the carriercomprises an RF signal.
 30. The system according to claim 23, whereinthe receiver further converts the carrier signal into an IF signal,wherein the IF signal is representative of the carrier signal.
 31. Thesystem according to claim 23, wherein the receiver further filters thecarrier signal to limit the bandwidth to a signal of interest.
 32. Thesystem according to claim 23, wherein the receiver further stores adigitized filtered signal in memory.
 33. The system according to claim23, wherein the signal processor further calculates an average basebandand an average bandwidth of the carrier signal.
 34. The system accordingto claim 23, wherein the signal processor further down-converts thedigital signal to the baseband of the carrier signal.
 35. The systemaccording to claim 23, wherein the signal processor further decimatesthe digital signal at twice the symbol rate.
 36. The system according toclaim 23, including removing a M initial samples of the error vector,wherein the M initial samples occur while the blind equalization anddemodulation is sill converging.
 37. The system according to claim 23,including removing a DC bias from the error vector.
 38. The systemaccording to claim 23, including receiving the signal, filtering thesignal and digitizing the filtered signal